Metal-to-ligand charge transfer MLCT states

We have reported the first direct observation of the vibrational spectrum of an electronically excited state of a metal complex in solution (40). The excited state observed was the emissive and photochemically active metal-to-ligand charge transfer (MLCT) state of Ru(bpy)g+, the vibrational spectrum of which was acquired by time-resolved resonance Raman (TR ) spectroscopy. This study and others (19,41,42) demonstrates the enormous, virtually unique utility of TR in structural elucidation of electronically excited states in solution. 2+... 

Fluorescent redox switches based on compounds with electron acceptors and fluorophores have been also reported. For instance, by making use of the quinone/ hydroquinone redox couple a redox-responsive fluorescence switch can be established with molecule 19 containing a ruthenium tris(bpy) (bpy = 2,2 -bipyridine) complex.29 Within molecule 19, the excited state of the ruthenium center, that is, the triplet metal-to-ligand charge transfer (MLCT) state, is effectively quenched by electron transfer to the quinone group. When the quinone is reduced to the hydroquinone either chemically or electrochemically, luminescence is emitted from the ruthenium center in molecule 19. Similarly, molecule 20, a ruthenium (II) complex withhydroquinone-functionalized 2,2 6, 2"-terpyridine (tpy) and (4 -phenylethynyl-2,2 6, 2"- terpyridine) as ligands, also works as a redox fluorescence switch.30... 

Metal-to-Ligand-Charge-Transfer (MLCT) States in M(H)(CO) (a-diimine) Complexes (M = Mn, Re)... 

The unresolved emission band takes its origin from a metal-to-ligand-charge-transfer (MLCT) state according to EHMO calculations [38]. This is not surprising because toe metal center is electron-rich (rf ) and toe isocyanide fragment exhibits Jt-acid empty orbitals. The absorption spectrum peaks placed at 204nm... 

Ru(bpy) " in Silica-Titania. Tris(2,2 -bipyridine)ruthenium(II), Ru(bpy)3 ,has received the considerable attention because of its unique properties such as strong luminescence, moderate excited-state lifetime, energy and electron transfer reactions and chemical stability (Kalyanasundaram, 1992). The luminescent excited state of Ru(bpy)3 isassigned to the metal-to-ligand charge-transfer (MLCT) state. The luminescence properties are very sensitive to polarity and viscosity of solvent because of the MLCT character. When Ru(bpy)3" is excited, solvent reorientation aroimd the excited state molecules occurs to stabilize the MLCT excited state with a large dipole moment. Therefore, a blue shift of the luminescence is induced when motion of the solvent molecules is restricted. 

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